Which of the following is NOT affected by a magnetic field?

A magnetic field interacts with certain materials and phenomena in specific ways. Magnetic materials, electric currents, and ferromagnetic substances are all influenced by magnetic fields.

Magnetic materials typically include those that can be magnetized and will align with an external magnetic field. Electric currents produce a magnetic field around them, which indicates a clear interaction with a magnetic field. Ferromagnetic substances, such as iron, are particularly responsive to magnetic fields due to their atomic structure, which allows them to become magnetized easily.

On the other hand, gases, particularly those that are not ionized, do not experience any significant effect from a magnetic field. This is largely because gases consist of neutral atoms or molecules that do not have magnetic moments or sufficient charge density to interact with the field. Therefore, while other options directly relate to magnetic effects, gases remain unaffected, making this the correct answer.